A work schedule sought by some U.S. air-traffic controllers to get three-day weekends produces severe fatigue because it’s impossible to get adequate rest between duty periods, a Congress-mandated study found.

More than 100 U.S. airport towers and radar rooms have so few flights that they should be shut down late at night under the government’s own guidelines, a move that would save taxpayers $10 million a year.

Moments before a single-engine aircraft and a helicopter collided over the Hudson River near Manhattan in 2009, an air-traffic controller who should have been advising the plane’s pilot was on the phone, joking with an airport worker about a dead cat.

U.S. aviation regulators don’t have enough investigators to examine close calls in the skies, which have increased more than 50 percent since 2009, according to a report by the Transportation Department’s Inspector General.

Customs inspectors trimmed working hours at the nation’s second-busiest container port and lines more than doubled at some of the largest airports as U.S. spending cuts began slowing transportation links.